The top tray temperature is taken as 3
F above the tower top temperature. The
vapor to the top tray is taken to be about 5
F above the top tray.
Condenser duty is 7,020,000 Btu/h.
Calculating the Reboiler Duty
It is important that the reboiler duty achieve a balance between generating an
effective rate of vapor for stripping the bottom product while maintaining an
economic vapor load on the stripping trays. As a rule of thumb, this can be achieved
with a stripping rate as a percent of bottom product of between 70 and 90 mol %.
The tower bottom temperature has already been calculated as 358
F at 138 psia.
The composition of the vapor in equilibrium at these conditions has also been
calculated. Thus the moles/h of bottom product are 1,359.79; then the moles of
strip-out required will be 1,359.79 Â 0.9 = 1,223.8 mol/h. The molecular weight of
the strip-out is 88 (from the equilibrium composition). The heat balance to determine the reboiler duty now follows. In this calculation the draw-off tray to the
reboiler is about 10
F lower than the tower bottom temperature, that is, 348
F
(Table 11).
Calculating the Overall Tower Heat Balance
Knowing the reboiler and condenser duties, an overall heat balance over the tower
can be calculated and will provide the preheat required in the feed and consequently
its temperature. A calculation to determine the bubble point of the feed at 134 psia
(mean inlet pressure) was made. This temperature was 305
F. Should the calculated temperature be above this, then an enthalpy curve would need to be developed
for the feed to determine its actual enthalpy and temperature. The overall heat
balance now follows (Table 12).
Enthalpy of the feed is
21, 713, 000
142, 659 ¼ 152:2 Btu=lb; from the enthalpy tables, the
temperature is found to be 276
F. The feed tray is based on a preliminary tower
Table 10 Tower top heat balance example
Stream
L/V
Mole wt
F
lb/h
Btu/lb
MMBtu/h
In
Ref o/flow
V
56.8
160
46,803
305
14.275
Distillate
V
55
160
10,494
306
3.211
Total in
57,297
17.486
Out
Dist Prod
L
55
100
10,494
150
1.574
Ref Liquid
L
56.8
155
46,803
190
8.893
Condenser
By difference
7.019
Total out
57.297
17.486
Distillation of the ‘‘Light Ends´´ from Crude Oil in Petroleum Processing
215
F above the tower top temperature. The
vapor to the top tray is taken to be about 5
F above the top tray.
Condenser duty is 7,020,000 Btu/h.
Calculating the Reboiler Duty
It is important that the reboiler duty achieve a balance between generating an
effective rate of vapor for stripping the bottom product while maintaining an
economic vapor load on the stripping trays. As a rule of thumb, this can be achieved
with a stripping rate as a percent of bottom product of between 70 and 90 mol %.
The tower bottom temperature has already been calculated as 358
F at 138 psia.
The composition of the vapor in equilibrium at these conditions has also been
calculated. Thus the moles/h of bottom product are 1,359.79; then the moles of
strip-out required will be 1,359.79 Â 0.9 = 1,223.8 mol/h. The molecular weight of
the strip-out is 88 (from the equilibrium composition). The heat balance to determine the reboiler duty now follows. In this calculation the draw-off tray to the
reboiler is about 10
F lower than the tower bottom temperature, that is, 348
F
(Table 11).
Calculating the Overall Tower Heat Balance
Knowing the reboiler and condenser duties, an overall heat balance over the tower
can be calculated and will provide the preheat required in the feed and consequently
its temperature. A calculation to determine the bubble point of the feed at 134 psia
(mean inlet pressure) was made. This temperature was 305
F. Should the calculated temperature be above this, then an enthalpy curve would need to be developed
for the feed to determine its actual enthalpy and temperature. The overall heat
balance now follows (Table 12).
Enthalpy of the feed is
21, 713, 000
142, 659 ¼ 152:2 Btu=lb; from the enthalpy tables, the
temperature is found to be 276
F. The feed tray is based on a preliminary tower
Table 10 Tower top heat balance example
Stream
L/V
Mole wt
F
lb/h
Btu/lb
MMBtu/h
In
Ref o/flow
V
56.8
160
46,803
305
14.275
Distillate
V
55
160
10,494
306
3.211
Total in
57,297
17.486
Out
Dist Prod
L
55
100
10,494
150
1.574
Ref Liquid
L
56.8
155
46,803
190
8.893
Condenser
By difference
7.019
Total out
57.297
17.486
Distillation of the ‘‘Light Ends´´ from Crude Oil in Petroleum Processing
215